A bridge near a Minnesota dam may collapse. Officials say they can do little to stop it

MINNEAPOLIS -- Rushing waters from the Blue Earth River have already left a trail of debris and destruction on the edges of a southern Minnesota dam that partially failed last week, but officials acknowledged Tuesday the structure most in danger may be the bridge that looms nearby.

The County Road 9 Bridge is at risk of crumbling, and officials said they have little recourse. The threat to the bridge accelerated after a bout of heavy rain and flooding pummeled the Midwest for days. The Blue Earth River's water levels rose dramatically and tested the structural integrity of the dam. The dam has held up, but the specter of collapse hasn’t waned.

Now, the roughly 40-year-old bridge locals use to commute across the dam from rural patches of land to nearby towns, may topple over if the weather doesn't cooperate.

“Unfortunately, we’re at the mercy of Mother Nature at this point," said Ryan Thilges, the public works director for Blue Earth County. “We’re very concerned about the potential for partial or full failure of the bridge.”

Thilges stood atop a hillside on the eastern side of the Rapidan Dam near the Minnesota city of Mankato. He was flanked by Minnesota Gov. Tim Walz and other officials who went to the dam to get an update on flood conditions and recovery efforts.

Officials are warily watching both the dam and the bridge, noting that the still-surging river has drastically changed the area.

“I think the concern is that is the bridge going to be structurally damaged by this and will it need to be replaced?” Walz said.

The floodwaters forged a new river channel around the dam cut deeply into a steep riverbank, toppling utility poles, wrecking a substation, swallowing a home and forcing the removal of a beloved store. The conditions have made it too dangerous for officials to get close enough for a thorough inspection of the bridge, but they have already identified troubling signs of damage.

The river is washing away large amounts of sediment, causing instability to the bridge's supporting piers, built atop sandstone bedrock. Officials have been able to stabilize at least one pier but said they haven't been able to get to the others.

Complicating matters was “a massive spike of trees that came down the river” on Sunday, Thilges said. The dead trees, a product of drought over the last several years, collided with the bridge, and some are hanging on the piers. The county has not been able to find contractors who felt safe enough to clear the debris.

"Nobody was willing to send out their operator and risk their operator's life to try to push those trees through," Thilges said.

Flooding has caused millions of dollars in damage to bridges, homes and roads across Minnesota, Iowa, Nebraska and South Dakota. The dam captured attention after officials initially said it faced an “imminent threat” of collapse.

The Rapidan Dam is over a century old, finished in 1910. While it was built to generate electricity, it has been damaged by several rounds of flooding in recent decades. An April 2023 assessment conducted by the National Inventory of Dams found Rapidan to be in poor condition, and officials have been studying the possibility of removing it.

A federal disaster declaration has been approved for Blue Earth County, and local officials said the additional resources will be critical for rebuilding efforts. But those projects could be complicated by a sensitive landscape where relief efforts can sometimes exacerbate decline, Thilges said.

“I’ll be perfectly honest, all the solutions we came up with had almost as bad or worse adverse impacts that could affect the dam stability further, or it could result in damage to the bridge or additional erosion,” he added. "We need Mother Nature to give us a break."